Change: Deprecate legacy index format

Support for the legacy index format used by restic before version 0.2.0 has
been depreacted and will be removed in the next minor restic version. You can
use `restic repair index` to update the index to the current format.

It is possible to temporarily reenable support for the legacy index format by
setting the environment variable
`RESTIC_FEATURES=deprecate-legacy-index=false`. Note that this feature flag
will be removed in the next minor restic version.

https://github.com/restic/restic/issues/4602
https://github.com/restic/restic/pull/4724
